A CMOS type image sensor generally includes pixels or photosites disposed in a matrix configuration. Each pixel includes a photosensitive area, generally a photodiode, configured to accumulate electrical charges as a function of the light that it receives, and a reading circuit for measuring the quantity of charge accumulated by the photodiode. The reading circuit includes a transfer transistor for controlling the transfer of the electrical charges accumulated in the photodiode to a reading node to which a reading transistor is connected. The pixel is therefore controlled in accordance with a cycle including an integration phase, a reading phase and a reset phase. During the integration phase, the photodiode accumulates electrical charges as a function of the light that it receives. The reading phase includes generating a signal corresponding to the quantity of electrical charges accumulated by the photodiode during the integration phase. The reset phase includes eliminating the electrical charges accumulated by the photodiode during the integration phase.
Two shutter modes are used in image sensors, namely a global shutter mode and a rolling shutter mode. In the global shutter mode, all the pixels of the sensor are addressed at the same time and therefore in the same integration, reading or reset phase. In the rolling shutter mode, only two rows of pixels of the image sensor are in the integration phase and the reading phase, respectively. By comparison with the rolling shutter mode, the global shutter mode therefore offers the advantage of supplying a more accurate image, the exposure time and the content of which are uniform.
The embodiments set forth herein are related to image sensors using the global shutter mode.
Imaging devices (still cameras, video cameras) generally have a dynamic range much lower than that generally encountered in the real world, as perceived by the human eye. The dynamic range may be defined as the ratio between the maximum luminous intensity of an image (in the lightest or most illuminated area of the image) and the minimum intensity of the image (in the darkest area of the image). To increase the dynamic range of the images obtained, a high dynamic range (HDR) mode has been proposed that includes capturing a plurality of successive images, with different exposure (or integration) times. A first image obtained with a long integration time makes it possible to acquire details relating to dark areas of a scene and a second image obtained with a shorter integration time makes it possible to acquire details relating to light areas of the scene. These two images are then combined into a composite image including details both of the light areas and the dark areas.
Two successive images may be obtained using the pixel circuit PX shown in FIG. 1. The pixel circuit PX includes a photosensitive element EP such as a photodiode, a transfer transistor TG for controlling the transfer of the charges accumulated by the photosensitive element EP to a reading node RN, and a reading circuit. The transfer transistor TG is turned on by a signal VG applied to its gate. The reading circuit includes a transistor T5, a read selection transistor T6 and a reset transistor TR. The gate of the transistor T5 is connected to the reading node RN. The reading selection transistor T6 is connected to a power supply VT via the transistor T5 and supplies a reading voltage VX when it is turned on by a signal RD applied to its gate. The reset transistor TR is connected between the reading node RN and a power source such as the source VT and is turned on by a reset signal RST applied to its gate to reset the reading node RN. The pixel circuit PX may also include a global reset transistor T1 connecting the photosensitive element EP to a voltage source VAB when it is turned on by a control signal VGAB. Before an image is captured, the transistor T1 resets the photosensitive element EP. Similarly, the transistor TR may reset the reading node RN outside reading phases.
To implement the HDR mode, the pixel circuit PX includes a transistor T2 between the transistor TG and the reading node RN and a charge storage region CS formed between the transistors TG and T2. The charge storage region CS is represented in FIG. 1 by a capacitor C1 connected between ground and the connection between the transistors TG and T2.
FIG. 2 is a timing diagram of an image capture by an image capture device employing the global shutter mode and the HDR mode. The image sensor includes a plurality of pixel circuits PX. At a time t0, the photosensitive elements EP and the reading nodes RN of the image sensor are reset by the transistors T1 and TR of each pixel circuit PX of the image sensor. As soon as the reset phase RS ends at the time t1, there begins a first integration phase EXP1 for acquisition of a first image. At the time t2 marking the end of the first integration phase EXP1, the charges accumulated by the photosensitive elements EP during the integration phase EXP1 are transferred to the storage regions CS by applying a pulse to the gates of the transistors TG of the pixel circuit PX. At the time t3 there begins a first phase RD1 of reading the pixel circuits PX. This reading phase is generally performed row of pixels by row of pixels by applying a pulse to the gates of the transistors T2 of the pixel circuits PX to transfer the electrical charges from the storage regions CS to the reading nodes RN of the pixel circuits PX. The transistors T5 and T6 of the pixel circuits PX then read the value of the pixels row by row of pixels. The reading phase RD1 terminates at a time t6.
To implement the HDR mode, a second integration phase EXP2 and a second reading phase RD2 are performed. The reading phase RD2 cannot be started until the first reading phase RD1 has ended, i.e., before the end time t6 of the first reading phase RD1. Charges also need to be transferred between the photosensitive elements EP and the storage regions CS before the second reading phase RD2, which cannot be done before the end of the first reading phase RD1. This charge transfer is therefore performed between the time t6 and a time t7. The second reading phase (of the same duration as the first reading phase RD1) may commence from the time t7 and ends at a time t8.
Assuming that the second integration phase EXP2 is performed between a time t5 and the time t6, a reset phase RS is performed before the integration phase EXP2, between a time t4 and the time t5. The duration of the reset phase RS and the integration phase EXP2 of the second image between the times t4 and t6 is generally less than the duration of the reading phase RD1 for the first image. As a result, the photons received by the photosensitive elements EP of the image sensor between the times t3 and t4 are not used to form an image. This absence of image capture between the times t3 and t4 may lead to the formation in the image of a fuzzy or duplicated object (“ghost” phantom effect) if this object is mobile in the scene captured by the image sensor.
It may therefore be desirable to improve the quality of an image obtained in a high dynamic range (HDR) mode, notably by reducing the effects of fuzziness or phantom effects when the scene to be capture includes moving objects.